During the production of semiconductor dice or chips, many semiconductor dice are formed together on a single wafer. The wafer is then cut to separate the individual dice. Each of these semiconductor dice should then be individually mounted onto a support surface for further processing by utilizing a die bonding process. Thereafter, electrical connections are created between the dice and external devices, and the dice are later encapsulated with a plastic compound to protect them from the environment.
In prior art die bonders utilized in the said die bonding process, each individual die is usually picked up by a bond arm from the wafer and then transported to a substrate to perform attachment of the die onto the substrate. FIG. 1 is an isometric view of a die bonder 100 of the prior art. The die bonder 100 generally comprises a die bond head 102 which has an air nozzle 104 for creating a suction force to pick up a semiconductor die 106 from a wafer platform 108 holding the die 106. The die 106 is then transported and bonded onto a substrate 110.
In order to place the die 106 correctly and accurately onto the substrate 110, visual alignment is conducted with a vision system, usually comprising a first optical system 112 located over the wafer platform 108 and a second optical system 114 located over the substrate 110. Visual alignment is performed to capture images of the die 106 on the wafer platform 108 and the substrate 110. Positioning of the bond head 102 and air nozzle 104 will be performed according to the image captured of the die 106, which references an alignment pattern or fiducial mark on the die 106 for this purpose. Since the alignment pattern or fiducial mark of the die 106 is located on an upper side of the die facing the first and second optical systems 112, 114, these die bonders 100 allow the first optical system 112 to capture an image of the die 106 before the bond head 102 is moved into position to pick up the die 106, and to adjust the orientation of the die 106 according to the image taken of the substrate 110, before placing the die 106 onto the substrate 110.
A problem with this approach is that any error or unexpected movement of the die 106 that may be introduced during the die pick-up process after the image of the die 106 has been captured cannot be corrected or compensated for by the vision system. For example, an attraction surface of the air nozzle 104 and the die 106 may not be co-planar as a result of this error. The co-planarity between the air nozzle 104 and the die 106 is one of the major concerns during the pick-up process. When such co-planarity is not achieved, the position and angle of the die 106 will likely be shifted and rotated during the pick up process. It would be desirable to perform visual alignment of a die 106 after it has been picked up by the bond head 102 and before placement onto a substrate 110 to avoid errors introduced during die pick-up.
Furthermore, said prior art die bonders 100 that recognize the die 106 and the substrate 110 using two individual optical systems at two different locations require calibration. Calibration is needed to relate the coordinates as viewed by the optical systems with the coordinates of a bond head table (not shown) controlling positioning of the bond head 102. Any calibration error will have a direct adverse effect on the accuracy of the coordinate relationship existing between the two optical systems 112, 114 and the bond head table, and this will affect die bonding placement accuracy.
Also, during machine operation, the temperature of the bonding apparatus typically varies. Hence, the bond head table and optical systems 112, 114 may be heated up by their respective motors when the machine is running fast and may be cooled down when the machine is running slow or stopped. Thermal expansion or contraction will change the aforementioned relationship established between the optical systems and bond head table during calibration, thus affecting the placement accuracy as well. Generally, the further the two optical systems 112, 114 are separated, the worse the calibration error. This is because the calibration method usually involves the use of the bond head table to translate a calibration pattern from one optical system to the other. The distance of this movement should thus be accurately defined. The longer the distance, the larger is the effect of thermal error and table motion error.
Some die bonders may instead employ a movable optical system which can move from the location of the wafer platform 108 to the location of the substrate 110. The advantage of this design is that the images of both the die alignment pattern and the substrate alignment pattern can be captured with just one optical system, so that calibration for two individual optical systems need not be performed. Nevertheless, any motion error of such a movable optical system that is introduced when the optical system is moving between the location of the platform 108 to the location of the substrate 110 will also affect the placement accuracy.
When handling dice that are large or long in length, it will generally be better to employ an optical system with a small field of view. This means that if the optical system cannot capture the image of the whole die, it will need to capture the alignment pattern at one corner of the die 106 first and then move to capture the alignment pattern at an opposite corner of the die 106. In this way, the die centre position and the angle of rotation of the die can be calculated. A small field of view will provide the vision system with a higher resolution and thus higher pattern recognition (PR) accuracy. However, the motion accuracy of the optical system will also play an important part in the overall placement accuracy during die bonding. As explained above, another potential source of error may be introduced if positioning of the optical system is not sufficiently accurate. It would be desirable to minimize motion of the optical system used for visual alignment to avoid inaccuracy introduced from motion of the optical system.